1. Field of the Invention
The present invention relates to a method for manufacturing a sensor element for use in a gas sensor, and particularly to a method for stabilizing the condition of an electrode.
2. Description of the Background Art
Conventionally, various gas sensors have been used for recognizing a concentration of a desired gas component in a measurement gas. For example, as a device for measuring a NOx concentration in a measurement gas such as a combustion gas, known is a NOx sensor having a sensor element which is formed using an oxygen-ion conductive solid electrolyte such as zirconia (ZrO2) (for example, see Japanese Patent Application Laid-Open No. 2006-284223 and Japanese Patent No. 3537983).
In sensor elements of gas sensors including the NOx sensors disclosed in Japanese Patent Application Laid-Open No. 2006-284223 and Japanese Patent No. 3537983, a concentration of a measurement object gas component (object component) is obtained by utilizing the fact that in a case where the measurement object gas component is decomposed at a measuring electrode by the catalytic activity thereof, the amount of oxygen ion occurring at that time is proportional to a current flowing in the measuring electrode and a reference electrode. To be specific, a concentration value of the object component is recognized as follows: a relationship (sensitivity characteristics, a concentration profile) between a concentration value and a current value (output signal value) in each individual sensor element is obtained in advance by using a mixed gas whose object component concentration is already known; and in an actual use, a measured current value is converted into a concentration value based on the sensitivity characteristics.
Therefore, it is ideal that the current value is zero in a case where the object component does not exist in the measurement gas. However, actually, oxygen originally existing in the measurement gas is, though removed prior to the decomposition of the object gas component, left in a small amount and is decomposed, so that a current slightly flows. Accordingly, it is normal that prior to use, a current value (derived from residual oxygen and the like) under a state where the object component does not exist is identified as an offset value, and a value obtained by subtracting the offset value from a current value obtained under a state where the object component exists is used as a current value which is proportional to a gas concentration.
The above-mentioned sensitivity characteristics are determined before each gas sensor is used (for example, before shipment), and normally dealt as fixed characteristics without being changed when the gas sensor is used afterward. This is based on the assumption that actual sensitivity characteristics do not vary during the use of the gas sensor. If the actual sensitivity characteristics change over time, the concentration value which is calculated based on the sensitivity characteristics determined at the time of shipment loses its reliability as the use of the gas sensor continues, and eventually the gas sensor loses a measurement accuracy set in its specification.
However, the condition of an electrode of a sensor element obtained after an aging process is not stabilized because, for example, the degree of oxidation during baking of the element and the degree of reduction during the aging in a rich atmosphere, which is performed after the baking, are not constant, and a rich component that is left within the sensor element after the aging is suddenly decomposed. In other words, this means that the catalytic activity of the measuring electrode varies among sensor elements before they are shipped. Inspecting the element characteristics and further determining the sensitivity characteristics under such an unstabilized state may cause an erroneous determination in which a sensor element that is actually a non-defective product is determined as a defective product. This results in a reduction in the production yield. In view of this point, a technique is already known in which the condition of an electrode is stabilized by performing, prior to inspecting the element characteristics, a pre-treatment process for driving the sensor element in advance for a predetermined time period in a mixed gas atmosphere that is similar to an actual usage environment (for example, see Japanese Patent Application Laid-Open No. 2011-145285).
The pre-treatment method disclosed in Japanese Patent Application Laid-Open No. 2011-145285 exerts an effect of stabilizing the electrode of the sensor element, but the effect is not always constant. Thus, there is a problem that an improvement in the production yield is limited. Additionally, there is also a problem that this method cannot always be regarded as a satisfactory method from the viewpoint of the productivity of the sensor element, because it is necessary to drive the sensor element in the mixed gas atmosphere though the time period thereof is merely about ten minutes. Although it is possible to perform the pre-treatment by an apparatus configured to inspect the element characteristics, this is not always preferable from the viewpoint of the productivity of the sensor element, because, in such a case, the inspection of the element characteristics cannot be performed during the time period of the pre-treatment. On the other hand, using a special apparatus for performing the pre-treatment causes a problem that the size of an apparatus is increased because the gas is used, resulting in an increased cost.